1. Field of the Invention
This invention relates to an apparatus for the combustion of oxidizable substances, suspended in a carrier gas, and comprises of a gas inlet and gas outlet, a burner to which a high velocity mixing pipe is connected, a primary combustion chamber, a heat exchanger consisting of heat exchange tubes placed around the high velocity mixing chamber which transports the unprocessed gas in counter-flow to the already incinerated gas. These tubes are bent at one end.
2. Description of The Prior Art
An appropriate device of this type is described in German DE No. 30 43 286, Oct. 22, 1981 and corresponding EP No. 0 040 690 Dec. 2, 1981. Here the heat exchange tubes are bent inwardly on the high-temperature-side of the apparatus, i.e. curved inwardly in the vicinity of the burner and connected into a drum which encompasses the burner concentrically. Admittedly, this design offers the advantage that, different expansions of individual tubes, caused by temperature, do not lead to damage during process dependent applications, such as cracking.
This, however requires a considerable manufacturing effort in order to weld the inwardly bent tubes to the drum, due to the fact that, the space between the tubes is small. The drum as such cannot contribute to the compensating of tube expansions occuring from temperature differentials, because its relatively large wall thickness is necessary to prevent contraction and distortion during welding, which would in turn subject the tube bends to undue stress and strain.
Furthermore, the heat exchange tubes do not effectively transfer heat over their entire length, because the hot exhaust gas does not impinge directly on the curved ends, but is deflected and diverted away, so that it misses these parts of the tubes.
Moreover, the occurrence of scale formation in the vicinity of the tube bends is inavoidable, because these bends are located within the hottest part of the appliance. Scale build-up however can cause increased erosion of the tube walls when the tubes are subjected to large rates of change of expansion in this region. Such rates of change of expansion are more often than not characteristic of a particular process.